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Chapter 4: Gases in the Air

Release by humans

Humans release all the gases featured in this lesson. From Wikipedia:

The average human releases 0.5 to 1.5 litres of flatus a day by flatulating 12 to 25 times. The primary constituents of flatulence are the non-odorous gases nitrogen (ingested), carbon dioxide (produced by aerobic microbes or ingested), and hydrogen (produced by some microbes and consumed by others), as well as lesser amounts of oxygen (ingested) and methane (produced by anaerobic microbes). Odours result from trace amounts of other components (often containing sulphur).

Serious stuff. Funny how it's the smallest portion (the sulphur-containing smelly components) that we take most notice of.


Hydrogen is a non-metal* element, symbol H, atomic number 1.

* At extremely high pressures at the same time as very high temperatures (~3,000 K) it changes to metallic hydrogen. Metallic hydrogen is believed to be present in tremendous amounts in the gravitationally compressed interiors of Jupiter and Saturn.

Hydrogen makes up 75% of the universe by mass, or 90% by number of atoms, but only about 1 ppm in the atmosphere (abut 1 cubic centimetre hydrogen per cubic metre of air - that's a cube of hydrogen 1 cm on each side in a cube of air 1 m across). Because it's so light it easily escapes to space.

Hydrogen gas is H2 (each molecule of hydrogen gas has two atoms of hydrogen in it) and is easily made by (for example):

  • Electrolysis of water.
  • Cracking of methane (CH4).
  • Reacting a metal with hydrochloric acid (HCl). Using sodium (Na) would give H2 gas and table salt (NaCl).
  • Mixing aluminium and chlorine bleach (don't try at home).
  • Etc.

Hydrogen burns to make water, H2O (in gaseous form).

2H2 + O2 → 2H2O + heat

Hydrogen-oxygen flames are nearly invisible to the naked eye, which is why space rocket exhaust is pretty transparent. The space shuttle's main engines run on hydrogen and so have almost transparent exhaust but the shuttle also has solid-fuel booster rockets on the sides, which are basically big fireworks. Because hydrogen burns so easily (in concentrations as little as 4% in air) care must be taken when using it for combustion.


Oxygen is a non-metal element, symbol O, atomic number 8.

Oxygen gas is O2 and forms about 21% of our atmosphere. While oxygen is very reactive, and oxygen can combine with almost all other elements, it often needs heat applied to do so. Otherwise, reactions take place in slow motion, like an apple slice going brown, or paper yellowing with age.

In a pure oxygen environment fires can burn out of control, but if the supplies of both oxygen and fuel are controlled, useful things like rockets and oxyacetylene torches can be made. Acetylene burns at 3,200 to 3,300 °C (error in text).

Ozone (O3) is a form of oxygen found throughout the atmosphere. In the upper atmosphere it's made by high energy electromagnetic radiation, and helps to block UV light. At ground-level ozone it's made by electric discharges (eg, electric motors, photocopiers, etc) and is regarded as a pollutant. It's one of the most toxic inorganic compounds known (inorganic means it doesn't contain either carbon or hydrogen) and is useful for purifying air and water (including swimming pools).

Ozone can be smelled at just 15 ppb (part per billion), or 15 cubic millimetres per cubic metre, and hence its name, from the Greek word for "smell". (This is three times the concentration of just-detectable H2S.)

Liquid O2 and solid O2 have a light blue colour. Liquid O2 is usually obtained by the fractional distillation of liquid air. Liquid and solid O3 are dark blue.

Although ozone is a pollutant at ground-level it's produced by devices described as "air purifiers" and "negative ion generators". Often many of these devices are not intended to make ozone (some are designed specifically to make it) they all make at least some ozone.

The photo above is one is for a car, available online for about $11 including shipping. It has two reactions on the packaging for the oxidation of two smelly gases, which smell of rotten eggs and rotten cabbage respectively. I'll leave it up to the reader to figure out where they might come from in a car.

2H2S + O3 → H2O + SO2

CH3SH + O3 → CH3OH + SO2

Perhaps it should be stated that hydrogen sulphide (H2S), methanethiol (methyl mercaptain, CH3SH), ozone (O3), methanol (CH3OH) and sulphur dioxide (SO2) are all toxic. However the methanol would be further oxidised thus:

CH3OH + O3 → CO2 + 2H2O

Ozone is highly toxic. See Wikipedia air ioniser and ozone generator for some cautions when considering the use of one of these devices. Being a small and inexpensive device it will use the coronal discharge method of ozone production. When used in air (instead of pure oxygen) as well as ozone it produces nitrogen oxides, which are also toxic (see under Nitrogen for more information).

N2 + O2 → 2 NO

N2 + 2 O2 → 2 NO2

Because liquid oxygen is more concentrated than gaseous oxygen it is a much better oxidiser and so is used for rocket propulsion, normally with liquid hydrogen. It was also used by George Goble for his liquid oxygen barbeque, for which he won the 1996 Ig Nobel award for chemistry. He explains how things went for "lighting of the grill with 3 gallons (11.3 L) of liquid oxygen. Started with 60 lbs (27 kg) of charcoal, and burnt up 40 lbs (18 kg) of it in 3 seconds. Result is a grill ready to cook in about 3 seconds, and all the old grease, etc burned off. Don't try this at home." He has also found that a charcoal briquette soaked in liquid oxygen will explode with about the energy of a stick of dynamite.

A recently discovered form of oxygen, tetraoxygen (O4), is a deep red solid that is created by pressurising O2 to the order of 20 GPa (about 200,000 atmospheres). Its properties are being studied for use in rocket propulsion and similar applications, as it is a much more powerful oxidiser than either O2 or O3.

Carbon dioxide

Carbon dioxide is a compound, symbol CO2.

CO2 content in fresh air varies between 0.03% (300 ppm) to 0.06% (600 ppm) depending on location and in exhaled air is about 4.5%. Breathing high concentrations of carbon dioxide is painful because the carbon dioxide dissolves in the mucus lining of the lungs giving carbonic acid. CO2 is not poisonous but it is suffocating - we can't live on it.

Carbon dioxide is a gas at room temperature, and forms a solid known as dry ice at -78 °C. When dry ice warms up, instead of melting it sublimates - it turns straight into a gas. CO2 can be a liquid at about five atmospheres and more, starting at -57 °C. CO2 gas is heavier than air, while dry ice is heavier than water.

CO2 is made when carbon combines with oxygen by combustion, respiration, etc. Plants use CO2 to form carbohydrates, while people use CO2 to make soft drinks, to power air guns, baking (where the CO2 is made by yeast or baking soda/sodium bicarbonate) and in some fire extinquishers.

In the 1750s the Scottish physician Joseph Black used limestone (calcium carbonate, CaCO3) to make a gas he termed "fixed air" which was denser than air and did not support either flame or animal life. He also worked out that the gas is produced by animal respiration and microbial fermentation.

In 1772, Joseph Priestley used carbon dioxide produced from the action of sulphuric acid on limestone to prepare soda water, the first known instance of an artificially carbonated drink.

Incomplete combustion of carbon results in carbon monoxide (CO), which is odourless and very toxic because it combines with haemoglobin in the blood preventing it from carrying oxygen around the body. The main (common) source of CO is car exhaust, although catalytic converters reduce the carbon monoxide in car exhaust gases considerably.


Nitrogen is a non-metal element, symbol N, atomic number 7.

Nitrogen gas is N2 and is the most common gas in the atmosphere, making up 78% of it. Nitrogen is a very reactive element but N2 is very stable (it's joined by a triple bond), making it quite inert.

Nitrogen was discovered in 1772 by Daniel Rutherford who called it "fixed air" - it won't support combustion or animal life.

At pressure nitrogen acts as an anaesthetic. In water each 10 metres of depth increases pressure by about 1 atmosphere, so divers cannot go below 30m breathing ordinary compressed air. Also, if divers surface too quickly they can get the "bends" - nitrogen bubbles forming in the blood.

However nitrogen is essential for life and is present in all living cells (eg, in DNA), amino acids, chlorophyll in plants, etc.

A Christian scientist in the USA, George Washington Carver, refused to believe that God would create anything useless, as the peanut was then thought to be. So he kept working with peanuts until he discovered that peanut plants (and all legumes) have bacteria that can fix nitrogen in the soil. (It has since been discovered the bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia.) And not content with that, he kept going and discovered more uses for peanuts. Encyclopedia Britannica says:

He ultimately developed 300 derivative products from peanuts - among them cheese, milk, coffee, flour, ink, dyes, plastics, wood stains, soap, linoleum, medicinal oils, and cosmetics - and 118 from sweet potatoes, including flour, vinegar, molasses, rubber, ink, a synthetic rubber, and postage stamp glue.

He improved peanut horticulture so much that he is considered by many to be the father of the peanut industry. More about peanut butter can be read on the Aqualab Origin of Peanut Butter page.

Nitrous oxide (N2O or laughing gas), is used as an anaesthetic, a propellent in whipped cream, and also has a use in car engines, allowng more fuel to be burned, giving greater power. The three ways it does this are (1) by breaking down at high temperatures to provide more oxygen, (2) by liquid N2O evaporating it cools the intake air - cooler air equals denser air - which means the compression stroke is easier, and (3) forming N2 gas on the power stroke, releasing lots of energy - see explosives below. It's not all fun and games, though. From Wikipedia:

One of the major problems of using nitrous oxide in a reciprocating engine is that it can produce enough power to destroy the engine. Power increases of 100-300% are possible, and unless the mechanical structure of the engine is reinforced, most engines would not survive this kind of operation.

Nitric oxide (NO) and nitrogen dioxide (NO2) are very toxic and are bad pollutants mostly produced by car exhaust. NO is short lived and reacts rapidly in air to make NO2 which has a brown colour and has "a characteristic sharp, biting odour." Concentrations as low as 4 ppm can anaesthetise the nose.

Air glow is a term for light emitted from the upper layers of the atmosphere of Earth, or of another planet. The glow is generally bluish in color. One mechanism that produces air glow occurs when an atom of nitrogen combines with an atom of oxygen to form a molecule of nitric oxide (NO). In the process a photon is emitted. This photon may have any of several different wavelengths characteristic of nitric oxide molecules. The free atoms are available for this process because molecules of nitrogen (N2) and oxygen (O2) are dissociated by solar energy in the upper reaches of the atmosphere, and may encounter each other to form NO.

For Nitrogen and Explosives see Chapter 13 under Explosives.


For more information about how common the constituent air gases are see this page.